COLLISION COURSE: Justin Turner (left) collides with left fielder Andrew Brown after making a catch during the first inning of the Mets’ 2-1 loss to the Nationals in Game 2 of their doubleheader. (
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WASHINGTON — Split happens.

LaTroy Hawkins could only think in those terms as he watched Ryan Zimmerman crush a fastball over the right-field fence in the ninth inning, ending the Mets’ hopes of a doubleheader sweep last night.

“It happens,” Hawkins said after the Mets’ 2-1 loss to the Nationals. “You don’t want to walk him right there. He hits the ball to the opposite field, you tip your hat to him.”

Jenrry Mejia dominated in the first game with seven shutout innings that led the Mets to an 11-0 victory. The righty, in his first start this season, impressed Mets brass enough he will remain with the team as a sixth starter, allowing Matt Harvey and Zack Wheeler to get extra rest as the summer progresses. To clear roster space, the Mets optioned Kirk Nieuwenhuis to Triple-A Las Vegas last night.

On the one-year anniversary of his major league debut, Harvey allowed an unearned run over eight innings in the nightcap and received a second no-decision in his last three starts.

The Mets (46-54) had moved to seven games below .500 for the first time since May 30 with the Game 1 victory, but took a step backward by leaving 10 runners on base in the nightcap. Included in the misery was a Juan Lagares pop out in the ninth with the go-ahead run at third base with one out.

“He’s got to get a ball he can handle,” manager Terry Collins said. “He was anxious and swung at a ball, it might have even been ball four.”

Harvey’s unearned run came in the fifth on Daniel Murphy’s throwing error. Justin Turner made a glove flip to Murphy for the force at second base on Wilson Ramos’ grounder, but Murphy’s throw to first was wild, allowing Jayson Werth to score the tying run.

“I just got caught up in [Turner’s] play and I threw it in the crapper,” Murphy said. “That was a tough one to swallow. I should have made the play.”

In his first major league appearance since last September, the 23-year-old Mejia allowed seven hits, struck out seven and did not allow a walk. Murphy carried the offensive load with a 4-for-5 performance that included two homers and five RBIs.

Mejia has battled elbow problems for most of the last three seasons — and may need surgery for bone chips after this year is complete — but his signature pitch, the cutter, has returned.

“I heard his ball moves like a DUI drunk, but I got a chance to see it today and I was very impressed,” Hawkins said.

If Mejia is all the way back, he joins a crop of young power pitchers, including Rafael Montero, Noah Syndergaard and Jacob deGrom, that could give the Mets layers of talent behind Harvey and Wheeler.

“[Mejia] was the rage in 2010 and everybody thought the sky is the limit and the injuries have set him way back,” Collins said. “I think today it showed [his name] should be in the mix.”

Mejia got stronger as the game progressed, retiring 11 of the last 12 batters he faced. Last season, Mejia appeared in five games (three starts) for the Mets, but said he never had a starter’s mentality.

“I threw like a reliever,” Mejia said. “Now I come here and my first pitch is like a starter, and I’ve really been excited about that.”

Murphy slapped a run-scoring single to left in the ninth inning that allowed him to match a career high with five RBIs. Later in the inning, Ike Davis hit a three-run homer. The blast was Davis’ first since June 2, eight days before he was demoted to Las Vegas.

Lagares and Murphy each delivered an RBI single in the seventh that extended the Mets’ lead to 5-0.

Murphy’s two-run homer in the third gave the Mets a 3-0 lead. Lagares began the rally with a two-out double before Murphy hit a shot just inside the right-field foul pole. It gave Murphy his second career two-homer game — his other came on June 27 of last year at Wrigley Field.Murphy’s 440-foot blast to right-center in the first gave the Mets their first run against Jordan Zimmermann (12-6)

“I really think it started with Mejia,” Murphy said. “He threw the ball so well today and to come up, first outing of the year like that and I think the biggest thing was to throw strikes like that, he really pounded the zone and got some big ground balls when he needed to.”